1. Field of the Invention
The present invention relates to magnetic sensors, magnetic sensor devices, and torque sensors. More specifically, the present invention relates to a magnetic sensor, a magnetic sensor device, and a torque sensor, which are used in, for example, an electric power steering apparatus installed in vehicles.
2. Description of the Related Art
As shown in FIGS. 28 and 29, conventionally, when variation in a rotation angle of an object is detected by a magnetic sensor without contacting the object, a gear 1 is used as the object, and the magnetic sensor 2 is used for generating a pseudo sine-wave (see FIG. 30). The rotational position of the gear 1 is determined by counting the number of pulses in the pseudo sine-wave. In order to improve the detection accuracy, the pseudo sine-wave is processed by a multiplication circuit. Typically, the magnetic sensor 2 includes two magnetosensitive elements 3a and 3b to obtain one signal. The magnetosensitive elements 3a and 3b are arranged in the rotating direction of the gear 1 with a gap therebetween, the size of the gap being half the size of the pitch between the teeth of the gear 1.
On the other hand, electric type power steering apparatuses have been developed for increasing the power to turn a steering wheel of a vehicle. In such apparatuses, a steering torque applied by a driver is detected, and an assisting force is generated in accordance with the detected torque by an electric motor provided in a steering mechanism.
With respect to sensors for detecting the steering torque, non-contact type torque sensors having a simple construction are preferably used from the viewpoint of reliability and cost. In conventional torque sensors, the steering torque is determined from a magnetostriction, variation in inductance, a sliding friction, etc., or by using a potentiometer, etc.
With respect to conventional magnetic sensors, however, the detection accuracy is limited by the resolution of pulses, and there is a problem in that variation in small rotation angles cannot be detected with sufficiently high accuracy.
In addition, in torque sensors which determine a torque from a sliding friction or by using a potentiometer, there is a problem in that abrasion of a slider and a resistor occurs at contacting parts thereof. In addition, there is also a problem in that, since the torque sensors are usually disposed near a driver, the driver feels discomfort by noises generated from the contacting parts of the slider and the resistor. In addition, in a case in which a torque is converted into a linear movement of a sliding member, there are parts which are mechanically in contact. Thus, there is a problem in that sufficient durability cannot be obtained due to abrasion of the contacting pares. Furthermore, with respect to other conventional torque sensors, there is also a problem in that, since a large number of machined components, some of which having a complex shape and requiring high dimensional accuracy, are necessary, a high cost is incurred.
Accordingly, an object of the present invention is to provide a magnetic sensor, a magnetic sensor device, and a torque sensor, which are able to detect a variation in a rotation angle with high accuracy even when the variation is significantly small, and of which the construction is simple.
In order to attain the above-described object, according to the present invention, a magnetic sensor, which is used for detecting a rotational displacement of a magnetic element disposed such that the magnetic element is inclined substantially linearly with respect to the rotating direction thereof, includes a plurality of magnetosensitive elements which are arranged in a direction perpendicular to the rotating direction of the magnetic element with a gap therebetween and face the magnetic element, at least parts of the respective magnetosensitive elements overlapping on end portions of the magnetic element in a direction perpendicular to the rotating direction of the magnetic element.
In addition, a magnetic sensor device according to the present invention includes a magnetic element which is provided on a surface of a rotating member such that the magnetic element is inclined substantially linearly with respect to the rotating direction of the rotating member; and a magnetic sensor for detecting a displacement of the magnetic element caused by the rotation of the rotating member, the magnetic sensor including a plurality of magnetosensitive elements which are arranged in a direction perpendicular to the rotating direction of the magnetic element with a gap therebetween and face the magnetic element, at least parts of the magnetosensitive elements overlapping on end portions of the magnetic element in a direction perpendicular to the rotating direction of the magnetic element.
Preferably, a distance between the magnetic element and the magnetosensitive elements is 0.3 mm or greater. In addition, the magnetosensitive elements are, for example, semiconductor magnetoresistive elements.
Since the magnetic sensor faces the magnetic element which is fixed to the rotating member, the magnetic sensor is mechanically independent from the rotating member and is able to detect the displacement of the magnetic element without contacting it. In addition, since contacting parts are not included in the mechanism, a completely non-contact detection is achieved. In addition, since the number of machine-processed components is small and constructions thereof are simple, the magnetic sensor device and the torque sensor are provided at low cost. In addition, the pseudo sawtooth wave having a broad linear region can be obtained as the output signal from the magnetic sensor, and small variation in the rotation angle can be detected with high accuracy.
In addition, by setting a gap between the magnetosensitive elements to be larger than a gap between the end portions of the magnetic elements, the resistances of the magnetosensitive elements vary more smoothly by the rotation of the magnetic element. In addition, by arranging the magnetosensitive elements such that all regions thereof face the magnetic element in the direction perpendicular to the rotating direction of the magnetic element, sensitivity of the magnetosensitive elements increases. In addition, by arranging the magnetosensitive elements such that approximately all of the regions thereof face the end portions of the magnetic element, linear regions in the pseudo sawtooth wave are made broader, and sensitivity of the magnetosensitive elements increases.
In addition, by disposing a plurality of magnetic elements in the rotating direction thereof, every time the rotating member is rotated by one turn, two or more periods of pseudo sawtooth wave is obtained from the magnetic sensor as the output signal. As a result, sensitivity of the output signal to small variations in the rotation angle is increased. Moreover, by counting the periods of the pseudo sawtooth wave, the torque sensor can also be used as a steering angle sensor.
In addition, according to the present invention, a torque sensor, which is used for detecting a torque applied between a plurality of shafts which are connected to each other via a torsion bar and are able to rotate relative to each other due to the torsional deformation of the torsion bar when the torque is applied, includes the above-described magnetic sensor or the above-described magnetic sensor device for each of the shafts.
When the torsion bar which is disposed between the shafts is not twisted, the voltage difference between the output signals obtained from the respective magnetic sensors which are individually provided for the shafts does not change. When the torsion bar is twisted, relative position between the magnetic elements which are individually provided on the rotating members varies, so that the voltage difference between the output signals varies in accordance with the applied torque. According to change in voltage difference between these output signals, difference of relative angle between the shafts is detected. Thus, the torque is calculated from the Young""s modulus of the torsion bar.
The torque sensor according to the present invention may include two or more magnetic sensors provided for each of the shafts. The phases of the respective output signals obtained from the magnetic sensors differ from each other by, for example, approximately 180 degrees, approximately 120 degrees, etc.
For example, two magnetic sensors may be provided for each of a plurality of shafts, and the two magnetic sensors may be disposed such that the phases of the respective output signals obtained therefrom differ from each other by approximately 180 degrees. Accordingly, in a case in which it is difficult to detect the voltage difference from the output signal obtained from one of the two magnetic sensors, the output signal obtained from the other one of the two magnetic sensors can be used for detecting the voltage difference. Thus, the torque can be detected with higher accuracy.
In addition, three magnetic sensors may be provided for each of a plurality of shafts, and the three magnetic sensors may be disposed such that the phases differ from each other by approximately 120 degrees. Accordingly, in a case in which it is difficult to obtain the output signal from one of the three magnetic sensors, the output signal can be obtained from either one of the other two magnetic sensors. Furthermore, when malfunction of one of the three magnetic sensors occurs, the one that is malfunctioning is easily identified. In addition, the electric power is not necessarily cut when the malfunction occurs, and detection of the torque and the steering angle can still be performed by the remaining two magnetic sensors for the time being.